1. Field of the invention
The present invention relates to coated color particles with extremely low susceptibility to decoloring, which are preferably used as particulate spacers for liquid crystal display cells. The present invention also relates to coated particles having excellent dispersibility and other desirable properties and therefore are highly appropriate for use as spacers for liquid crystal display cells, as standard particles for various measurements, as carriers for diagnostic tests, etc.
2. Description of the prior art
Inorganic or organic particles are widely used as particulate spacers in liquid crystal display cells, labelling materials such as fluorescent particles for flow cytometry in the biochemical field, standard particles in applications such as measurement of grain sizes by electron microscopy, carriers for immunological diagnostic reagents in diagnostic testing, etc.
As such particles, colored particles are used as required. For example, the colored particles are preferably used for spacers in liquid crystal display cells. Such colored particles are generally obtained by a coloring treatment of core particles with dyes or pigments. Known methods of obtaining such particles include, for example, those described in Japanese Laid-Open Patent Publication Nos. 57-189117 and 59-24829.
However, the colored particles obtained by the methods of the aforesaid Laid-Open Patent Publications are susceptible to loss of the coloring agent during use. For example, if colored particles of the aforesaid conventional types are exposed to water, various solvents, liquid crystals, acids or bases, then the coloring agent may be eluted from the colored particles, and hence nonuniformity of color appears, or in some cases complete decolorization of the particles occurs. Moreover, soiling or contamination of other objects or substances by the coloring agent eluted from the colored particles also constitutes a problem. Heretofore, no effective method for prevention of the decoloring of such colored particles has been proposed.
Particulate spacers used for liquid crystal cells are placed between two glass substrates and maintain a constant gap between the said pair of glass substrates. Since this gap must be maintained at a fixed width over the entire region of the said glass substrates, a large number of the particulate spacers are disposed at prescribed intervals between the substrates. In general, since the particulate spacers are made of fine particles, they are prone to aggregate, resulting in the formation of clumped aggregates of from several to several tens of particles. If particulate spacers are situated between the glass substrates in such an aggregated state, then the gap precision deteriorates. Furthermore, in the transmission type liquid crystal cells of black-and-white displays, such portions of the displays usually appear white, thereby lowering the contrast of the said black-and-white displays.
In order to improve the dispersibility of particulate spacers for liquid crystal cells, Japanese Laid-Open Patent Publication No. 62-195623 discloses a method that comprises a process of applying ultrasonic wave to the particulate spacers, thereby improving the dispersibility of the said spacers. However, the method requires a special apparatus, and is extremely impractical for industrial applications. The most effective method for this purpose would be the prevention of mutual aggregation of the particles by appropriate processing of the particles from an industrial viewpoint; however, no specific method of implementing such a process has yet been proposed in the prior art.